In recent years, the amount of information transmitted through the optical fiber communication network has rapidly increased due to the progress in the optical fiber communication system, especially due to the invention of the erbium-doped fiber amplifier (EDFA) and the dense wavelength division multiplexing (DWDM) system. In order to meet the demand for increased data capacity, research and development to increase the number of wavelengths to be multiplexed is being carried out on a modulation method in which the frequency efficiency is high. In the DWDM system, there is a need for an optical component having a more advanced function, such as a chromatic dispersion compensator which compensates the wavelength dispersion and the dispersion slope of each channel more precisely than the dispersion compensation optical fiber module used in the related art. In addition, research and development is also being carried out on a variable chromatic dispersion compensator, which is capable of meeting a rerouting or a temporal and periodical change of the dispersion characteristic of the optical transmission line, and a polarization mode dispersion compensator, which compensates the polarization mode dispersion dynamically.
On the other hand, as the sizes of information communication systems and the number of information communication systems installed increase rapidly, the large amount of power consumed by a computer system or a high-end router becomes a problem from the points of view of not only economic efficiency but also environmental impact. For this reason, green ICT (Information and Communication Technology) for reducing the power and environmental impact is required. If various transmission apparatuses, such as a router, can be made smaller, the apparatus accommodation efficiency in the data center or the central office of a telecommunication carrier will be improved. As a result, the space use efficiency is improved. In addition, it becomes possible to greatly reduce the power consumed by the air conditioner of the data center or the central office, which contributes to energy saving. Accordingly, there is also a demand for power reduction and miniaturization of optical components used in various optical transmission devices.
As a technology for manufacturing small and highly functional optical components, a silicon photonics technology of manufacturing an optical waveguide device by using a CMOS manufacturing process has come into the spotlight and research and development on this are being carried out. By forming the optical waveguide using a high refractive index material, such as silicon (Si) or silicon nitride (SiN), it becomes possible to miniaturize the conventional optical waveguide device which uses the silica (SiO2)-based glasses as the main constituent material of the core and cladding. In addition, since a semiconductor material obtained by doping an impurity dopant into Si is used, it becomes possible to adjust the refractive index by applying the voltage from the outside. As a result, a device with a variable optical characteristic can be realized. Moreover, since it is a manufacturing process suitable for mass production, it is expected that the price of the optical component will drop in the future.
As a known planar optical waveguide device having a Bragg grating pattern, a uniform-pitch grating structure shown in FIG. 46 is known, in which the pitch PG of a fin 201 and a valley 202 provided on a sidewall of an optical waveguide 200 is constant. Moreover, as shown in FIG. 47, the chirped-pitch grating structure is known, in which the pitch of a fin 301 and a valley 302 provided on a sidewall of an optical waveguide 300 gradually varies like PGi>PGj>PGk>PGl>PGm>PGn.
Patent Document 1 discloses a wavelength dispersion compensation device in which Bragg gratings having one certain period are formed in the optical waveguide, such as an optical fiber or a waveguide formed on a substrate (planar optical waveguide), and the sampling structure is formed in the optical waveguide so as to overlap the Bragg gratings and which performs wavelength dispersion compensation in a plurality of wavelength channels. The sampling structure is formed by a phase sampling pattern with one certain period which is longer than the period of the Bragg gratings. Each period of the phase sampling is divided into a plurality of space regions in the direction along the optical axis of the optical waveguide, and the phase of the Bragg gratings changes discontinuously on the boundary where the adjacent space regions are in contact with each other. As shown in FIGS. 1A to 1D of Patent Document 1, there is no discontinuous change of the phase in one space region.
In addition, Non-patent Document 1 is a scientific paper written by the inventors of Patent Document 1 and discloses technical information which complements Patent Document 1. First, the Bragg grating pattern of a single channel at the center wavelength is designed by using the knowledge of Patent Document 1. The grating pattern is derived from the spectral characteristic of desired reflection and wavelength dispersion by using the inverse scattering method. However, in the Fiber Bragg Grating, there is a limitation on the range where the refractive index can be changed to form the grating pattern. For this reason, an operation of performing the inverse Fourier transform of the spectral characteristic and apodizing it so that the limitation is not exceeded is added. Thus, the pitch of the Bragg gratings in the pattern obtained changes continuously with the position. Then, the Bragg grating patterns of a plurality of channels are designed by phase sampling. In the Fiber Bragg Grating, the phase sampling is effective because there is a limitation on the variation range of the refractive index.
Patent Document 2 discloses a technique for realizing a device such as a chromatic dispersion compensator, which has a complicated optical characteristic, by solving the inverse scattering problem to design and manufacture the planar optical waveguide device.
The resolution of the photolithography process in each technology node of the CMOS device manufacturing technology is not necessarily determined only by making shorter the wavelength of light of the light source of the exposure apparatus, but is also improved by introducing Resolution Enhancement Techniques (RET), such as the phase shift mask (PSM). In the technology node of 400 nm or more, the light source which emits i-line beam with a wavelength of 365 nm was used. In each technology node of 250 nm, 180 nm, and 130 nm, the KrF excimer laser with a wavelength of 248 nm was used. Currently, the ArF excimer laser with a wavelength of 193 nm is introduced, and the immersion exposure technique has been developed. These developments therefore enabled technology nodes of 90 nm, 65 nm, and 45 nm to be practically used.
The phase shift method is known in the related art as a method of improving the resolution limit in the reduction projection exposure method in which the scanner (stepper) is used. According to Non-patent Document 2, the resolution limit in the phase shift method is improved about twice in comparison with the exposure method in which a usual transmission mask is used.
By now, not only a modulator or a light emitting/receiving element but also various optical passive components, such as a photonic crystal waveguide, a silicon wire waveguide, and an AWG have been studied using silicon photonics technology as the optical components for optical fiber communication systems. Although an active cable module using a silicon photonics transceiver module has already been commercialized, the study of the silicon photonics technology is still in its infancy. A lot of studies up to now have been done by using a direct write process using an electron beam (EB) apparatus. Accordingly, knowledge on the photolithography process using a photomask has not been sufficiently accumulated yet. In manufacturing a silica glass based planar optical waveguide of an early date with a relative refractive index difference (usually called Δ) of about 0.3%, a one to one photomask was able to be used since the core width of the optical waveguide was as large as 7 μm. In contrast, in the high relative refractive index difference optical waveguide manufactured by using the silicon photonics technology, the effective refractive index for the signal light is increased. Accordingly, the core size of the single-mode optical waveguide is reduced to one-severalth, or to one several tenth thereof and the specific distance of the periodical structures of the photonic crystal waveguide or grating optical waveguide also becomes very small. For this reason, a finer process technology is required.
On the other hand, in the optical waveguide device, sufficient thickness or depth is required to realize the thickness of the core of the optical waveguide or to form the surrounding structure, such as cladding, unlike the LSI in which electronic circuit devices, such as a DRAM and a CPU, are integrated. For this reason, when the surrounding structures are formed, a case often occurs where the latest fine process cannot necessarily be applied and the old technology node, such as thick-film resist application, needs to be used. Moreover, in the cases of optical components for optical fiber communication systems which have less volume demand than more established ICs such as the DRAMs, CPUs, and the like: using the industrial 12-inch wafer fabrication line processes for mass production does not necessarily lead to cost reduction. Rather, in order to reduce the cost, it helps to manufacture an appropriate number of optical components by the old process using 6-inch wafers or 8-inch wafers in many cases. For example, the silicon photonics optical waveguide device for optical fiber communication systems manufactured using the 130 nm technology node is disclosed in Non-patent Document 3. The 130 nm technology node is the process in which the scanner (stepper) that uses a wavelength of 248 nm, for example, is used and the phase shift mask is used to improve the resolution.